Interest in metallocene and non-metallocene single-site catalysts (hereinafter all referred to as single-site catalysts) has continued to grow rapidly in the polyolefin industry. These catalysts are more reactive than conventional Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of .alpha.-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Unfortunately, the uniformity of the molecular weight distribution of polyolefins made with single-site catalysts reduces their thermal processing ability. It is difficult to process these polyolefins under the conditions normally used for Ziegler-Natta polymers. The lower processing ability limits the development of single-site catalyst-based polyolefins because altering process conditions often requires a large capital investment.
Another disadvantage of single-site catalysts is low thermal stability. High temperature is preferred in solution and supercritical olefin polymerization processes, particularly toward the end of the reaction, because high temperature drives the polymerization to completion and reduces the viscosity of the final product. Low viscosity is needed because the polymer is often transferred and treated to remove catalysts, residual monomers, or solvents. High temperature, however, deactivates single-site catalysts.
Furthermore, single-site catalysts usually need a large amount of an alumoxane activator. The alumoxane complicates the olefin polymerization process and leaves high aluminum residues if not removed from the polymer. An important disadvantage of alumoxanes is that the large amounts typically present deactivate Ziegler-Natta catalysts that are used after or simultaneously with a single-site catalyst in an olefin polymerization.
A method for improving thermal processing ability of polyolefins is known: U.S. Pat. No. 5,236,998 discloses a parallel multiple reactor process for producing a blend of polyethylene and a copolymer of ethylene and a long-chain .alpha.-olefin using a Ziegler-Natta catalyst. The polymer blend has a broad molecular weight distribution, and therefore, it has improved thermal processing ability. U.S. Pat. No. 5,747,594 discloses a two-stage polymerization process. In a first reactor, ethylene and an .alpha.-olefin are polymerized with a metallocene catalyst. The polymerization continues in a second reactor with a Ziegler-Natta catalyst. An alumoxane activator is used in the first reactor. However, we have found that using an alumoxane activator with a single-site catalyst in the first reactor can kill a Ziegler-Natta catalyst in the second reactor, particularly when a highly reactive, thermally stable Ziegler-Natta catalyst (for example, a mixture of VOCl.sub.3 and TiCl.sub.4) is used.
Improved olefin polymerization processes are needed. A valuable process would sidestep the thermal stability problems of single-site catalysts and would avoid alumoxane activators. An ideal process would give olefin polymers with both good physical properties and excellent processing ability.